Ensuring confidentiality of information managed by a computer system is a very significant issue for a company, or the like, that possesses the information. A technology for encrypting data stored in a storage device is known as a technology for improving the confidentiality of information. Japanese Laid-open Patent Publication No. 2005-107855, Japanese Laid-open Patent Publication No. 2006-302259 and Japanese Laid-open Patent Publication No. 2004-201038 described below disclose technologies for encrypting data stored in a storage medium such as a magnetic disk. Generally, when the data stored in the storage medium is encrypted, the state of progress of the encryption process is managed by a controller that controls the encryption process.
If information for managing the state of progress of the encryption process is lost during the encryption process due to, for example, failure of the controller, it is impossible to distinguish between the data that has been encrypted and the data that has not been encrypted among the data stored in the storage medium. In this case, the encryption process cannot be resumed or the data cannot be correctly decoded; therefore, the data cannot be restored.